1. Field of the Invention
The present invention relates to a sensor utilizing attenuated total reflection (hereinafter referred to as ATR), such as a surface plasmon resonance sensor for quantitatively analyzing a substance in a sample by utilizing the generation of surface plasmon, and more particularly to a sensor, utilizing ATR, which detects a dark line generated in a measuring light beam due to ATR by the use of photodetection means.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, compression waves called plasma waves are generated. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon resonance sensors for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that surface plasmon is excited by light waves. Among such sensors, one employing a system called xe2x80x9cKretschmann configurationxe2x80x9d is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance sensor employing the xe2x80x9cKretschmann configurationxe2x80x9d is equipped mainly with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on a surface of the dielectric block, for placing a sample thereon; a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at the interface between the dielectric block and the metal film and that ATR can occur at the interface by surface plasmon resonance; and photodetection means for detecting the state of the surface plasmon resonance, that is, ATR, by measuring the intensity of the light beam satisfying total internal reflection at the interface.
In order to obtain various angles of incidence as described above, a relatively thin light beam may be caused to strike the above-mentioned interface at different angles of incidence, or relatively thick convergent or divergent rays may be caused to strike the interface so that they include components incident at various angles. In the former, the light beam whose reflection angle varies with a change in the incidence angle of the incident light beam can be detectedby a small photodetector that is moved in synchronization with the incidence angle change, or by an area sensor extending in the direction in which the angle of reflection varies. In the latter, on the other hand, rays reflected at various angles can be detected by an area sensor extending in the direction in which all of the rays can be received.
In the surface plasmon resonance sensor mentioned above, if a light beam strikes the metal film at a specific incidence angle xcex8sp equal to or greater than an angle at which total internal reflection occurs, evanescent waves having electric field distribution are generated in the sample in contact with the metal film, whereby surface plasmon is excited in the interface between the metal film and the sample. When the wave vector of the evanescent light is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light satisfying total internal reflection at the interface between the dielectric block and the metal film drops sharply. The sharp intensity drop is generally detected as a dark line by the above-mentioned photodetection means.
Note that the above-mentioned resonance occurs only when an incident light beam is p-polarized. Accordingly, a light beam must be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from an incidence angle xcex8sp at which ATR takes place, the dielectric constant of the sample can be obtained by the following Equation:
Ksp(xcfx89)=(xcfx89/c){xcex5m(xcfx89)xcex5s}xc2xd{xcex5m(xcfx89)+xcex5s}xc2xd
where Ksp represents the wave number of the surface plasmon, xcfx89 represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and xcex5m and xcex5s represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant xcex5s of the sample is known, then the density of the specific substance within the sample can be derived based on a predetermined calibration curve or the like. As a result, the incident angle xcex8sp at which the aforementioned reflected light intensity drops can be known, thereby the properties relating to the dielectric constant, that is, the refractive index of the sample can be derived.
As a similar sensor making use of ATR, a leaky mode sensor is disclosed, for instance, in xe2x80x9cSpectral Researches,xe2x80x9d Vol. 47, No. 1 (1998), pp. 21 to 23 and pp. 26 and 27. The leaky mode sensor is constructed mainly of a dielectric block in the form of a prism, for example; a cladding layer formed on a surface of the dielectric block; an optical waveguide layer, formed on the cladding layer, for placing a sample thereon; a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at the interface between the dielectric block and the cladding layer and that ATR can occur at the interface by the excitation of an optical waveguide mode in the optical waveguide layer; and photodetection means for detecting the excited state of the waveguide mode, that is, ATR by measuring the intensity of the light beam satisfying total internal reflection at the interface.
In the leaky mode sensor with the construction mentioned above, if a light beam falls on the cladding layer through the dielectric block at angles of incidence equal to or greater than an angle at which total internal reflection occurs, the light beam is transmitted through the cladding layer and then only light with a specific wave number, incident at a specific angle, is propagated in the optical waveguide layer in a waveguide mode. If the waveguide mode is excited in this manner, the greater part of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light satisfying total internal reflection at the above-mentioned interface drops sharply. Since the wave number of light propagating in the optical waveguide layer depends on the refractive index of the sample on the optical waveguide layer, the refractive index of the sample and/or the properties of the sample related to the refractive index can be analyzed by finding the above-mentioned specific incidence angle at which ATR occurs.
In the conventional surface plasmon resonance sensor and leaky mode sensor mentioned above, a laser is generally employed as the light source. Particularly, if a single mode laser is employed, the curve for ATR changes sharply and therefore a measurement can be made with high sensitivity. However, the emission wavelength of the laser is susceptible to influences from the outside and easily fluctuates. Because of this, there is a problem that it will become difficult to make a measurement with a high degree of accuracy. That is, if the emission wavelength of the laser fluctuates, it will have detrimental effects on the condition for generating surface plasmon (or the condition for exciting a waveguide mode) and cause noise to occur in a detection signal (which represents the intensity of light satisfying total internal reflection at the interface between the dielectric block and the thin film layer), resulting in a reduction in the accuracy of measurement.
Hence, to avoid the aforementioned problem, there has been proposed an apparatus employing a light-emitting diode (LED) as a light source. The LED has a great spectral line width and is not affected by wavelength fluctuation. However, since the spectral line width is too great, there is a problem that spectral sensitivity is low. In addition, the LED has the following disadvantages: since the light receiving area is large, angular resolution for ATR is low; and since light emitted from an LED is not linearly polarized light, a polarizing plate, etc., must be used and therefore the power of the measuring light is reduced.
The present invention has been made in view of the circumstances mentioned above. Accordingly, it is the primary object of the present invention to provide a sensor, utilizing ATR, which is capable of making a measurement with a high degree of accuracy.
To achieve this end and in accordance with an important aspect of the present invention, there is provided a sensor utilizing attenuated total reflection, comprising:
a dielectric block;
a thin film layer, formed on a surface of the dielectric block, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer; and
photodetection means for detecting the attenuated total reflection by measuring the intensity of the light beam satisfying total internal reflection at the interface;
wherein a semiconductor light emitting element that emits light by super radiance is employed as the light source.
In accordance with another important aspect of the present invention, structured particularly as a surface plasmon sensor, there is provided a sensor utilizing attenuated total reflection, comprising:
a dielectric block;
a metal film, formed on a surface of the dielectric block, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at an interface between the dielectric block and the metal film; and
photodetection means for detecting the attenuated total reflection that results from surface plasmon resonance by measuring the intensity of the light beam satisfying total internal reflection at the interface;
wherein a semiconductor light emitting element that emits light by super radiance is employed as the light source.
In accordance with still another important aspect of the present invention, structured particularly as a leaky mode sensor, there is provided a sensor utilizing attenuated total reflection, comprising:
a dielectric block;
a cladding layer formed on a surface of the dielectric block;
an optical waveguide layer, formed on the cladding layer, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at an interface between the dielectric block and the cladding layer; and
photodetection means for detecting the attenuated total reflection that results from excitation of a waveguide mode in the optical waveguide layer by measuring the intensity of the light beam satisfying total internal reflection at the interface;
wherein a semiconductor light emitting element that emits light by super radiance is employed as the light source.
The referent of the phrase xe2x80x9cemits light by super radiancexe2x80x9d is emission of light in which induced emission occurs but not emission of light by laser oscillation, due to the reflective structure of the element (refer to Japanese Unexamined Patent Publication Nos. 11(1999)-74559, and 9(1997)-232180, etc.).
A facet emission LED and a super luminescent diode (SLD) are examples of a semiconductor light emitting element that emits light by super radiance. The basic device structure of an SLD is similar to that of a semiconductor laser. However, the reflectance ratio of the facet thereof is low, in the order of less than 1%, and selectivity of the single mode is low, so light having a broad spectral width is emitted. Accordingly, it is a device that emits light having a broad spectral width like that of an LED, from a small facet like that of a semiconductor laser. For example, a super luminescent diode (AS2C211) manufactured by the Anritz Corporation is a point light source having a light emitting facet of 4 xcexcmxc3x971 xcexcm, but its spectral line width is 17 nm.
Notice that the above-mentioned sensors utilizing ATR are capable of employing an area sensor, a line sensor, etc., as the photodetection means. More specifically, the sensors can suitably employ a two-piece photodiode, a photodiode array, etc.
In the sensors of the present invention utilizing ATR, a semiconductor light emitting element that emits light by super radiance is employed as a light source that emits a light beam. The SLD is a point light source similar to a semiconductor laser. Because of this, high angular resolution is obtained in the measurement of ATR, and a fluctuation in the emission wavelength (mode hop), which is causative of a reduction in the accuracy of measurement when a semiconductor laser is employed, does not occur. Thus, the present invention is capable of preventing noise that occurs in a measurement signal because of a fluctuation in the emission wavelength. As a result, a sample can be analyzed with a high degree of accuracy. In addition, the spectral line width is narrow compared with light-emitting diodes (LEDs), and high sensitivity is obtained compared with an apparatus employing an LED as a light source. Since the light beam is linearly polarized, the present invention is capable of eliminating the use of a polarizing plate, etc., required of LEDs. As a result, there is no power loss and sensitivity can be enhanced. Furthermore, in the case where a semiconductor layer is employed as a light source, the light beam is coherent and therefore there are cases where a reduction in the signal-to-noise ratio (S/N ratio) due to coherent noise becomes a problem. However, in the present invention, the light beam emitted from the SLD is incoherent, so that interference noise is less liable to occur. Thus, the S/N ratio can be enhanced.
That is, in the sensors of the present invention utilizing ATR, the semiconductor light emitting element that emits light by super radiance is employed as a light source, whereby sufficiently high accuracy of measurement is realized compared with the case where an LED or semiconductor laser is employed as a light source.